Method for producing composite material for coating applications

ABSTRACT

The production of solid lubricant agglomerates by combining solid lubricant powder, an inorganic binder, other fillers if optionally desired, and a liquid to form a mixture, and driving off the liquid to form dry agglomerates which are subsequently classified by size or milled and classified by size to yield agglomerates of a desired size range. These agglomerates are then treated to stabilize the binder, thereby strengthening the binder and rendering it nondispersible in the liquid. The undesired size ranges can be readily recycled because the agglomerates with untreated binder can be reprocessed, thereby promoting high recovery rates.

BACKGROUND OF THE INVENTION

(i) Field of the Invention

This invention relates to a method for the production of solid lubricantagglomerates and, more particularly, to a method for the production ofrounded hexagonal boron nitride agglomerates within a desired size rangeand of a predetermined composition and density.

(ii) Description of the Related Art

Solid lubricants such as hexagonal boron nitride (hBN) powders areblended or clad with matrix forming metal and blended with otheradditives to form thermal spray compositions used as abradable seals ongas turbine engines, turbochargers, steam turbines and other rotaryequipment. Such an application is, for instance, described in U.S. Pat.No. 5,976,695 by K. Hajmrle et al. Among other thermal sprayapplications using solid lubricant compositions, is protection ofcompressor disc and blades against fretting as described in U.S. Pat.No. 5,601,933 by K. Hajmrle et al.

For thermal spray and other applications, larger particles than thoseresulting from conventional hBN production techniques are necessary. Byits nature, the synthesis of hBN produces particulates smaller than 10microns whereas thermal spray applications require larger particles inthe range of 20 to 150 microns.

It is well known in the art to produce solid lubricant particles of hBNby hot pressing fine particles of hBN to form large agglomerates andsubsequently crushing, milling and classifying the resulting particlesto the desired particle size range. This process is expensive due to thehigh cost of the several manufacturing steps, especially the hightemperature hot pressing step. The cost of the product is furtherincreased because of the production of undersize which must be discardedas waste, and oversize particles which must be further crushed andprocessed. The undersize fine fraction lowers the process recoverysubstantially.

The final product produced by the “hot pressing” method is relativelysoft which causes problems in further processing. For instance, whensuch material is further processed by hydrometallurgical metal claddingin an autoclave, the particles disintegrate to a high degree and theparticle size of the final product is difficult to control. Strongerparticles are required in this application. Stronger particles are alsoan advantage in powder mixes when two or more different powders aremechanically blended. The soft particles prepared by the hot pressingmethod have to be handled more gently.

The low recovery of the hot pressing process for producing large hBNparticles results in high cost product. The narrower the requiredparticle size cut, the higher the scrap rate and the product cost. Forthat reason, compromises must be made and wider particle size cuts usedthen those required for a particular process or product. Exact tailoringof particle size is impractical due to high hBN cost. These compromiseslead to inefficiencies in subsequent thermal spraying such as lowdeposit efficiency and low retention of hBN in the spray coating.

Another disadvantage of prior art hBN particles is the angular andirregular shape of the particles, which may cause problems in powderfeeding during thermal spraying.

A further disadvantage of the prior art hBN particles is theinflexibility in composition and density of the particles. For instance,addition of fillers, i.e. other solid particles, to hBN composition islimited by the hot pressing process that is carried out at hightemperatures and pressures.

It is therefore a principal object of the present invention to provide acost-effective method for the production of strong, rounded, solidlubricant particles, within a desired and, if required, narrow sizerange.

It is another object of the invention to provide a method for theproduction of rounded agglomerate particles such as hBN based,molybdenum disulphide based, or graphite based agglomerates which isreliable and simple in operation and which produces essentially no wastematerial.

Another object of the invention is the provision of a method for theproduction of rounded solid lubricant agglomerates having a variety ofdesired compositions without changing process parameters, whilepermitting production of uniformly dense or porous product particles.

Yet another object of the invention is the provision of a method for theproduction of solid lubricant agglomerates containing other fillers anda binder to create new composite particle compositions. Fillers cancomprise any solid particles such as particulate polymers, ceramics andmetals.

SUMMARY OF THE INVENTION

These and other objects of the invention can be achieved by the methodof the invention which, in its broad aspect, comprises combiningparticulate solid lubricant, an inorganic binder, and a liquid toproduce a workable mixture. Fillers can also be added to the solidlubricant-binder-liquid mixture to lower the cost of the final productor to enhance its properties.

The above mixture, which may have high or low viscosity, is then driedby evaporating the liquid to form solid agglomerates. These agglomeratesmay have the required particle size for the particular application, suchas when the drying is carried out by spray drying or, when theagglomerates are large, the agglomerates can be crushed to achieve therequired particle size. In all cases the agglomerates are classified.Then, the oversized fraction can be re-crushed to achieve the requiredparticle size if shape is not important, or mixed with the liquid andreprocessed. The undersized fraction can be mixed with the liquid andreprocessed.

In a preferred embodiment of the method of the invention, theparticulate solid lubricant is mixed with the binder in a weight ratioof about 19:1 to 1:19 of solid lubricant to binder, preferably in aweight ratio range of about 9:1 to 4:6 of solid lubricant to binder andmore preferably in a weight ratio of about 8:2 of solid lubricant tobinder, and mixed with water to produce a slurry in the range of about 5to 60 weight % solids, preferably in the range of about 20 to 30 weight% solids. The slurry can be spray dried to form rounded agglomerates.The binder is preferably hydrophilic and may be selected from the groupconsisting of bentonite, fillers earth, montmorillonite and the likehydrous aluminum silicates.

Although the spray drying process is the preferred way of producing theproduct, other processes can be used. Heavy, viscous mixtures can beproduced in large quantities (thousand cm³ or more), then dried andcrushed. Another method would be drum pelletizing followed by crushingand sizing. Yet another method would be to extrude a “spaghetti-like”mass of solid lubricant-filler-binder-liquid followed by drying,crushing and sizing.

The binder can be either solid, liquid such as sodium silicate, orliquid slurry, or any combination thereof.

The filler can be one or more particulate solid material that can lowerthe cost of the product or improve the product properties, such asparticulate polymers, ceramics or metals, or combination thereof. Thefiller can be added in an amount up to 40 volume % of the solids.

The solid lubricant is at least one of hexagonal boron nitride,graphite, calcium fluoride, magnesium fluoride, barium fluoride,tungsten disulfide and molybdenum disulphide powder, preferablyhexagonal boron nitride or molybdenum disulphide powder.

One major advantage of the invention is that by choosing a binder of theinvention the process achieves almost 100% recovery, even when extremelynarrow particular particle sizes of the product are required. This hastwo substantial advantages: the cost of solid lubricant, such as hBN, isvery high and therefore any gain in recovery significantly lowers thecost of the final product. Although spray drying, for example, is a lowcost process, its influence on the cost of production is much less thanthe cost of hBN. The second advantage is due to the fact that narrowparticle size cuts can be produced that are tailored perfectly tothermal spray process requirements. The results are much higher and morecontrollable deposit efficiency and subsequently lower cost of appliedcoatings.

The binder used in the process of this invention is preferablyinorganic. Organic binders do not stand up well to the thermal sprayprocess temperatures in which the flame temperatures are always above2000° C. When the organic binder is burned, the agglomerated particledisintegrates and the thermal spray process becomes uncontrollable.Inorganic binders are very stable at high temperatures and, for thatreason, more suitable for the thermal spray applications. The mostsuitable inorganic binders are those that have to be stabilized attemperatures higher than the drying temperature of the wet mixture. Thelow temperature processes for drying the mixtures contemplated in thisinvention do not stabilize the binder and the particles that falloutside the usable range can be readily reprocessed by redispersing thebinder and solid lubricant and any filler in the liquid. Only after theright particle size cut is produced is the binder stabilized andrendered non-dispersible and ready for thermal spraying. Stabilized inthis context means that the binder can no longer be redispersed in theliquid.

BRIEF DESCRIPTION OF THE DRAWINGS

The method of the invention will now be described with respect to theaccompanying drawings, in which:

FIG. 1 is a flowsheet illustrating a preferred embodiment of the methodof the invention; and

FIG. 2 is a schematic illustration of a spray drying system embodimentused in the method of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, a preferred embodiment of the method of theinvention is illustrated in which solid lubricant particles such as hBNparticles, which may be crushed hBN or fine hBN particles having a sizesmaller than −325 mesh (45 micron), are blended with a binder typifiedby bentonite powder in a weight ratio of about 1:19 to 19:1 of hBN tobinder, preferably in a weight ratio of about 9:1 to about 4:6 of hBN tobinder, and more preferably in a weight ratio of about 8:2 hBN tobinder, and slurried in water in mixing vessel 10 to provide a slurry ofabout 5 to 60 wt % solids in water, preferably about 20 to 30 wt. %solids in water, and transferred to tundish 11.

The water-solids slurry with the solids uniformly suspended therein canbe atomized into droplets in spray chamber 12 into which heated dryingair is passed. The water is evaporated from the droplets and the solidsproduct collected continuously from the chamber. The binder glues thefine hBN particles together and agglomerates are formed.

A typical spray drying chamber, detailed in FIG. 2, comprisescylindrical vessel 14 having conical bottom 16, into whichliquids-solids slurry is fed at top inlet 18, heated drying air fromblower 20 and heat exchanger 21 is fed tangentially at side inlet 22,drying air is exhausted at lower tangential outlet 24, and dried productfraction 25 is discharged at bottom apex outlet 26. Exhausted drying airpasses through cyclone 28 for collection of a fines fraction 30. Thisfines fraction is added to the recycle stream 36.

In FIG. 1, product 25 is fed to first screen 32 for removal of oversize34 which is added to the recycle stream 36. Undersize 38 from screen 32is fed to second screen 40 for discharge of oversize as a first productcut 42 and undersize 44 fed to third screen 46 for discharge of secondproduct cut 48. Undersize particles 50 from screen 46 are added to therecycle stream 36. The recycle stream 36 is added to mixing tank 10 insuch a proportion to facilitate continuous operation and full recovery.Product cuts 42 and 48 are passed to oven 52 and heated to at least 850°C. to stabilize the binder thus rendering the particles refractory andstrong and insoluble in water, the stabilized binder having thecharacteristics of insoluble ceramics.

In the event use of the agglomerate product in thermal sprayapplications does not require immediate stabilization, unstabilizedproduct can be thermally sprayed onto a surface and the heat of thermalspraying stabilize the binder or the binder can be stabilized by theheat during the use of the coating at elevated temperatures.

In a typical operation, the spray dried product would be passedsequentially to a 100 mesh (149 micron openings) screen 32, a 200 mesh(74 micron openings) screen 40 and a 325 mesh (45 micron openings)screen 46, with the plus 100 mesh oversize fraction and the −325 meshundersize fraction diverted to the recycle stream 36. The −100 +200 meshfraction and the −200 +325 mesh fraction are collected as separateproduct cuts and heated in oven 52.

The production cost of the material is controlled by the cost of rawmaterials and the cost of manufacturing the product. Both costs are verylow which makes this method very attractive from the cost point of view.The cost of fine hBN, for instance, is many times lower than that ofcoarse hBN. Also, the fine hBN particles are available from many lowcostsources as compared to coarse hBN particles available from only very fewproducers. The other major advantages are the very low cost of producingthe spray dried particles compared to the high cost of the “hotpress-crush-screen” approach and the high material recovery, usuallyhigher than 95% and in many cases close to 100%.

Although the description has proceeded with reference to the use ofbentonite as a binder and hBN as the solid lubricant particles to beagglomerated by the binder, it will be understood that the preferredbinder is hydrophilic and is typified by bentonite, fuller's earth andmontinorillonite which are hydrous aluminum silicates. Although it isunderstood that we are not bound by hypothetical considerations, it isbelieved the binders of hydrous aluminum silicates which permitreversible dehydration allow dispersion of the solid lubricant particlesin water upon recycle of undersize and oversize agglomerates to water inmixer 10

Solid lubricants other than hBN can be used alone or mixed with hBN andthe binder to reduce costs or add other properties to the product. Othersolid lubricants are at least one of graphite, calcium fluoride,magnesium fluoride, barium fluoride, tungsten disulfide and molybdenumdisulphide particles.

In some thermal spray applications, high amounts of porosity in theproduct are desirable. This can be achieved by adding a fugitivematerial such as a consumable polymer to the composition to provide anagglomerate product with increased porosity. A particulate polymerpowder such as polyester of a size smaller than 325 mesh can be blendedin an amount of about 1 to about 40% volume of the composition with thesolid lubricant and binder powder, preferably in an amount of about 10to 25%, in vessel 10 (FIG. 1). The fugitive polymer subsequently isvaporized or burned out to yield a porous, low density, open-cellproduct.

The method also provides flexibility to allow for the addition of otherfiller materials to either lower the cost or to improve the propertiesof the product. For example, particulate metals and ceramics such asalumina or quartz can be added to the mixture.

The stablized product is suitable for hydrometallurgical processing suchas by metal alloy cladding, or the product can be blended with metalalloys such as CoCrAIYSi alloys for thermal spraying to produceabradable seals or anti-fretting coatings.

It will be understood that other embodiments and examples of theinvention will be readily apparent to a person skilled in the art, thescope of the invention being defined in the appended claims.

1. A method for the production of solid lubricant agglomeratescomprising mixing particulate solid lubricant and an inorganic binder inthe ratio of about 19:1 to about 1:19 by weight and adding a liquid toproduce a mixture having about 5 to 60 weight % solids, drying themixture to produce dry agglomerates, and classifying by size or millingand classifying by size the dry agglomerates to obtain a desiredparticle size cut.
 2. A method as claimed in claim 1, in which anundersize particle fraction and an oversize particle fraction ofagglomerates are produced by classifying followed by reprocessing theundersize agglomerate fraction by redispersing the agglomerate in theoriginal liquid and reprocessing the oversize agglomerate particlefraction either by redispersing in the original liquid or by crushingand recovering the particle size that is in the desired particle sizecut.
 3. A method as claimed in claim 2 wherein, after the desiredparticle size cut is obtained by classifying, the binder in the desiredparticle size cut is rendered non-dispersible in the original liquid. 4.A method as claimed in claim 3, in which the solid lubricant is at leastone lubricant selected from the group consisting of hexagonal boronnitride, graphite, calcium fluoride, magnesium fluoride, bariumfluoride, tungsten disulphide and molybdenum disulphide particles.
 5. Amethod as claimed in claim 4, adding a filler to the solid lubricant andbinder in an amount up to 40 volume % of the solids.
 6. A method asclaimed in claim 4, in which the solid lubricant is hexagonal boronnitride and is mixed with the binder in a weight ratio of about 9:1 to4:6 hexagonal boron nitride to binder.
 7. A method as claimed in claim6, in which the liquid is water and the binder is hydrous aluminumsilicate that is stabilized at temperatures above 850° C. in the desiredparticle size cut.
 8. A method as claimed in claim 7, in which thebinder is at least one of bentonite, fuller's earth or montinorillonite.9. A method for the production of rounded, solid lubricant agglomeratescomprising mixing particulate solid lubricant and an inorganic binder ina weight ratio in the range of about 19:1 to about 1:19 of solidlubricant to binder in water in a mixer to produce a slurry having abouta 5 to 60 weight % solids, and drying droplets of the slurry to formdry, rounded, solid lubricant agglomerates.
 10. A method as claimed inclaim 9 comprising sizing the solid spherical agglomerates to produce anoversize fraction, an undersize fraction and a product fraction, pulpingand recycling the oversize and undersize fractions to the mixer, andrendering the product fraction non-dispersible in the original liquid.11. A method as claimed in claim 10, in which the solid lubricant is atleast one lubricant selected from the group consisting of hexagonalboron nitride, graphite, calcium fluoride, magnesium fluoride, bariumfluoride, tungsten disulfide and molybdenum disulphide particles.
 12. Amethod as claimed in claim 11, in which the solid lubricant is mixedwith the binder in a weight ratio of about 9:1 to 4:6 solid lubricant tobinder.
 13. A method as claimed in claim 12, in which the binder ishydrous aluminum silicate that is stabilized at temperatures above 850°C. in the desired particle size cut.
 14. A method as claimed in claim 9,in which the solid lubricant is hexagonal boron nitride and is mixedwith the binder in a weight ratio of about 9:1 to 4:6 hexagonal boronnitride to binder powder.
 15. A method as claimed in claim 11, in whichthe binder is at least one of bentonite, fuller's earth,montmorillonite, or combination thereof.
 16. A method as claimed inclaim 9, in which the solid lubricant is hexagonal boron nitride and ismixed with the binder powder in a weight ratio of about 9:1 to 4:6hexagonal boron nitride to binder powder and slurried with water toproduce a slurry containing about 20 to 30 weight % solids.
 17. A methodas claimed in claim 10, in which the solid lubricant is hexagonal boronnitride and is mixed with the binder in a weight ratio of about 9:1 to4:6 hexagonal boron nitride to binder powder and is slurried with waterto produce a slurry containing 5 to 60 weight % solids, and in which thebinder powder is selected from the group consisting of bentonite,fuller's earth, montmorillonite and combinations thereof.
 18. A methodas claimed in claim 10, in which the solid lubricant is hexagonal boronnitride, is mixed with the binder in a weight ratio of about 8:2hexagonal boron nitride to binder powder and slurried with water toproduce a slurry containing about 20 to 30 weight % solids, and in whichthe binder is hydrous aluminum silicate.
 19. A method as claimed inclaim 18, in which the hydrous aluminum silicate is at least one ofbentonite, fuller's earth or montmorillonite.
 20. A method as claimed inclaim 2, in which the binder is sodium silicate.
 21. A method as claimedin claim 17, additionally comprising adding a filler to the solidlubricant and binder in an amount up to 40 volume % of the solids. 22.Solid lubricant agglomerates produced by the method of claim
 8. 23.Rounded solid lubricant agglomerates produced by the method of claim 9.24. Rounded solid lubricant agglomerates produced by the method of claim17.
 25. Solid lubricant agglomerates produced by the method of claim 5.26. Rounded solid lubricant agglomerates produced by the method of claim21.